Soil penetrating tool assembly

ABSTRACT

A soil penetrating tool assembly (10) having a tool shank (18) which is pivotally mounted so as to clear obstructions in the ground. Movement of the shank away from its normal attitude is opposed by a hydraulic ram (22). The force required to be exerted by the shank (18) against the opposing force of the ram (22) is initially high but is reduced upon a predetermined movement of the tool (18) and thus a predetermined retraction of the ram (22). This allows the tool (18) to more easily clear large obstructions without causing damage to the tool (18).

TECHNICAL FIELD

This invention relates to an improved soil penetrating tool assembly,and in particular to an assembly which has the capability of avoiding orjumping obstructions, for example stumps in the ground.

BACKGROUND ART

Soil penetrating tool assemblies are commonly used in the agriculturalindustry to break up soil to facilitate sowing of seeds or for othersoil preparation procedures. The tool assemblies often include a shankwhich is sharpened at one end or alternatively provided with a soilpenetrating tool and the shanks are mounted to a cultivator frame thatis pulled by a tractor through a draw bar or mounted to a tool bar thatis attached to the tractor by means of a three point hitch.

It has been common for the shanks of the tool assemblies to bereleasably mounted to prevent damage to the shanks, frame, tool barand/or the shank carrying implement when an obstruction is encounteredin the soil.

Various devices have been used to attach tool assemblies and shanks to acultivator frame. In one common configuration, a clamping assembly isclamped or bolted to the frame and the shank supported in a yoke to theclamping assembly by a mounting pin or bolt and a shear pin extendingthrough sets of aligned holes. Where excessive obstructions or loads inthe ground are encountered, the shank may pivot and cause the shear pinto break. In such arrangements and in other arrangements where shear pinsystems are used, the breakage of a shear pin necessitates stopping theprime mover and replacement of the shear pin to enable ploughing to berecommenced.

Other arrangements have provided a spring system wherein a springapplies a biasing force to the shank to maintain the shank in theground. The spring, however, normally is required to be of such a sizeas to exert as high as possible force on the shank to maintain it in theground. Sufficient force to maintain the digging point in the ground inhard compacted soils would require springs of huge and impracticalsizes.

Other systems have used hydraulic rams as a means for opposing movementof the shank out of the ground. These systems would require huge andimpractical hydraulic rams to maintain penetration in hard compactedsoils. Such large rams or large springs also subject the digging pointsand shanks to possible breakage during the obstruction jumping actionwhen the shanks and points are in the raised non digging configuration.Furthermore, springs and rams in the above configurations are arrangedto exert a force on the shank in such a manner that all movements of theshank will be opposed by the same force.

The present invention aims to overcome or alleviate one or more of theabove described disadvantages or at least provide an alternative to thesoil penetrating tool assemblies of the type described above. Otherobjects and advantages will become apparent from the followingdescription.

SUMMARY OF THE INVENTION

The present invention thus provides in one aspect a soil penetratingtool assembly, said tool assembly including:

a soil penetrating tool arranged for movement between a lowered normaloperating position and a raised position wherein said tool can pass anobstruction,

fluid ram means associated with said tool, said ram means comprising acylinder and piston means movable in said cylinder,

fluid pressure relieving means associated with said ram means, saidfluid pressure relieving means normally resisting displacement of fluidfrom said ram means and thereby movement of said piston means relativeto said cylinder and movement of said tool from a said operatingposition, said fluid pressure relieving means permitting movement ofsaid piston means relative to said cylinder and thus said movement ofsaid tool when pressure of fluid within said ram means, consequent uponsaid tool encountering an obstruction, exceeds a predetermined maximumpressure, and

valve means internally of said ram means, said valve means connectingsaid fluid ram means to a fluid supply having a pressure lower than saidpredetermined maximum pressure when said movement of said piston meansexceeds a predetermined movement whereupon said tool is permitted tomove towards a said raised position to pass said obstruction.

The valve means suitably includes a first valve part fixed againstmovement relative to the cylinder and a second valve part movable withthe piston means. For supply of fluid from the fluid supply, a fluidsupply tube may extend longitudinally into the cylinder, the valve meanscontrolling communication between the tube and cylinder.

The cylinder of the ram means may be connected to an end housing at theend of the cylinder opposite the piston means. The end housing may beconnected to the fluid supply and support the fluid supply tube whichsuitably comprises an elongated tubular member which communicates withthe fluid pressure supply and which extends preferably coaxially throughthe cylinder and into the piston means. The free end of the tubularmember may carry the first valve part of the valve means. The secondvalve part of the valve means adapted for cooperation with the firstvalve part may be arranged on the piston means, the valve parts normallycooperating to prevent fluid flow through the tubular member into thecylinder. Upon the predetermined movement of the piston means relativeto the cylinder, the first and second valve parts separate to connectthe cylinder through the tubular member to the reduced pressure of thefluid supply. The piston means is thus more easily permitted to moveinto the cylinder thus permitting movement of the tool away from itsnormal operating positions. The valve means defined by the first andsecond valve parts closes to close communication between the tubularmember and cylinder when the piston means returns the tool back towardsan operating position.

The first valve part is preferably in the form of an annular member orenlargement at the free end of the tubular member. The piston and pistonrod of the piston means are preferably hollow such that the tubularmember extends into the interior thereof. The second valve part carriedby the piston means is preferably in the form of an annular insertwithin the piston and/or piston rod which in the closed position of thevalve means is in sealing engagement with the first valve part at theend of the tubular member. The annular insert may be formed of plasticsmaterial. When subject to fluid pressure within the cylinder, theannular insert may expand so as to ensure sealing engagement with thevalve part at the end of the tubular member.

The initial resistance to movement of the piston means is preferablyestablished through a fluid pressure multiplier. A similar effect may beachieved using a spring loaded valve. The fluid pressure multiplier mayinclude a stepped piston whose larger piston end is exposed to thesystem fluid pressure and whose smaller end normally communicates withthe cylinder such that when the piston means of the ram means isinitially retracted, its movement is opposed by the multiplied fluidpressure at the smaller end of the piston.

The soil penetrating tool of the tool assembly is preferably mounted forpivotal movement between its first and second attitudes. The toolassembly most commonly may be constructed so as to be mountable to atool bar of an implement and for this purpose the tool assembly mayinclude a clamp for mounting of the assembly to the tool bar. The toolmay be supported to the tool bar through a linkage which in addition tothe pivotal movement of the tool described above permits the tool tolift upwardly when striking or meeting an obstruction. The linkage maycomprise a pivotal link assembly which is pivotally mounted at one endto the clamp and which pivotally supports at its opposite end the shankof the tool. The fluid ram means may be connected between the tool shankor an extension thereof and the clamp to normally maintain the tool inthe ground in an operating position. The ram means may be pivotallyconnected to the clamp and to the shank. The ram cylinder suitably ispivotally connected to the clamp and the piston assembly of the rampivotally connected to the tool or an extension thereof. Alternativelythese connections may be reversed.

The ram means and associated fluid pressure relieving means permits thetool to undergo a first range of movement in its operating positions.This will occur when the load on the tool due to varying resistance inthe ground is sufficient to move the piston means within the cylinderovercoming the relief pressure of the fluid pressure relieving means. Onencountering a large obstruction in the ground however, a sufficientforce will be applied to the tool tending to cause the tool to pivot inorder to clear the obstruction with this movement being opposed by thefluid ram means until the fluid pressure therein exceeds thepredetermined pressure as set by the fluid pressure relieving means. Apredetermined movement of the tool and thus a predetermined movement ofthe piston means relative to the cylinder causes the valve means toconnect the ram means to the fluid supply of lower pressure which allowsthe tool to more easily pivot upwardly to clear the obstruction. Thismay be achieved through a gradual pressure reduction from the firstpredetermined pressure to the lower pressure or may be a steppedpressure reduction.

The reduced fluid pressure within the cylinder due to its connection tothe fluid supply also serves to urge the tool back towards its normaloperating position. During movement of the piston means urging the toolback towards its operating position the valve means will close such thatthe tool in its normal operating positions is again required to overcomethe increased pressure set by the fluid pressure relieving means andmove the predetermined distance before the ram means is connected to thelower pressure fluid supply to allow it to move clear of largeobstructions as before.

For return of the piston means to its fully extended position with thetool in an operating position, fluid is supplied primarily through thetubular member and open valve means into the cylinder. When the valvemeans is closed upon the predetermined extension movement of the pistonmeans causing the valve parts again coming into register with eachother, fluid to the cylinder can no longer be supplied through thetubular member. In this instance fluid to the cylinder is suppliedthrough an auxiliary passage connected to the system fluid supply. Toreduce shock loadings as the tool returns towards an operating positionthe auxiliary passage may be reduced in cross-section or include aportion of reduced cross-section to limit the flow to the cylinder andthereby reduce the speed of movement of the piston means as it returnsthe tool to an operating position.

Whilst the tool may be mounted through a linkage assembly as describedto a tool bar it may if desired be directly pivotally mounted to theclamp which clamps the tool assembly to the tool bar. The tool assemblyof course may be mounted to a prime mover by any other suitablearrangement for example through a three point linkage. The tool may beof any suitable type as required to achieve ploughing or other earthworking and may include a fixed tip or removable tip.

In yet a further aspect, the present invention provides a soilpenetrating tool assembly, said tool assembly including a soilpenetrating tool, support means for said tool, linkage means pivotallyconnected to said tool and said support means for supporting said toolfor movement between a lowered normal operating position and a raisedposition wherein said tool can pass an obstruction, fluid ram meanspivotally connected between said tool and said support means, said rammeans comprising a cylinder and a piston means movable in said cylinder,fluid pressure relieving means associated with said ram means, saidfluid pressure relieving means normally resisting displacement of fluidfrom said ram means and thereby movement of said piston means relativeto said cylinder and movement of said tool from a said operatingposition, said fluid pressure relieving means permitting movement ofsaid piston means relative to said cylinder and thus said movement ofsaid tool when pressure of fluid within said ram means, consequent uponsaid tool encountering an obstruction, exceeds a predetermined maximumpressure, and valve means internally of said ram means, said valve meansconnecting said fluid ram means to a fluid supply having a pressurelower than said predetermined maximum pressure when said movement ofsaid piston means exceeds a predetermined movement whereupon said toolis permitted to move towards a said raised position to pass saidobstruction.

In yet a further aspect, the present invention provides a fluid ramassembly, said ram assembly including a cylinder and a piston meansmovable in said cylinder, fluid pressure relieving means resistingdisplacement of fluid from said fluid ram assembly and thereby resistingmovement of said piston means in said cylinder from a first positionunless pressure of fluid within said ram assembly exceeds apredetermined maximum pressure, and valve means internally of said rammeans, said valve means connecting said ram means to a fluid supplyhaving a pressure lower than said predetermined pressure upon apredetermined movement of said piston means in said cylinder from saidfirst position to allow a less restricted movement of said piston meansin said cylinder away from said first position.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood and put intopractical effect, reference will now be made to the accompanyingdrawings which illustrate a preferred embodiment of the invention andwherein:

FIG. 1 illustrates a tool assembly according to the present invention ina first operative attitude;

FIG. 2 illustrates the tool assembly of FIG. 1 with the shank of thetool partially raised to clear an obstruction;

FIG. 3 illustrates the tool assembly of FIG. 1 with the shank assemblyraised further to clear an obstruction;

FIG. 4 is a sectional view of the ram for use in the tool assembly ofthe invention;

FIG. 5 is a perspective cut-away view of the end block of the cylinderof the ram of FIG. 4;

FIG. 6 is a schematic longitudinal sectional view of the block and partof the cylinder in the direction A--A of FIG. 5 with the multiplieromitted;

FIG. 7 is a sectional elevational view along line B--B of FIG. 6;

FIG. 8 is a sectional view along line C--C of FIG. 6;

FIG. 9 is a schematic sectional view in the direction B--B of FIG. 6with details to the right of line D--D omitted; and

FIG. 10 illustrates the ram of FIG. 4 in the pressure release attitude.

DETAILED DESCRIPTION OF THE EMBODIMENT

Referring to the drawings and firstly to FIGS. 1 to 3 there isillustrated a soil penetrating tool assembly 10 according to the presentinvention, including a tool support member 11 which is adapted to bemounted to a tool bar 12 and which for this purpose includes a firstfixed jaw 13 which is arranged to engage a pair of adjacent faces of thetool bar 12. A clamping plate 14 is adapted to co-operate with the toolbar 12 and member 11 to clamp the tool assembly 10 to the tool bar 12.The support member 11 pivotally supports a link assembly 15 through apivot connection 16. The link assembly 15 at its opposite end supportsvia a pivot connection 17 a tool shank 18 which may carry at its lowerend a soil penetrating tool 19 which may comprise a removable tip.

The shank 18 includes a pair of spaced ears 20 and a similar pair ofspaced ears 21 are provided on the support member 11. Extending betweeenthe respective pairs of ears 20 and 21 is an hydraulic ram 22 which isadapted to maintain the shank 18 in the ground. The ram 22 includes apiston assembly 23 pivotally mounted at 24 to the ears 20 and a cylinder25 terminating in a valve block 26 pivotally mounted at 27 between theears 21. Thus hydraulic fluid supplied to the cylinder 25 will extendthe piston assembly 23 to maintain the shank 18 in the ground. If anobstruction or increased loading is encountered by the shank 18, a forcewill be applied to the shank 18 tending to pivot it upwardly andrearwardly as shown in FIG. 2 and causing inward movement of the pistonassembly 23 into the cylinder 25 against the pressure of fluid in thecylinder 25. If the force applied by the shank 18 causes the pistonassembly 23 to be moved a predetermined distance into the cylinder 25,fluid pressure in the cylinder 25 is partially relieved to allow forfurther freer movement of the shank 18 upwardly in the manner shown inFIG. 3 to clear the obstruction to prevent the possibility of damage tothe shank 18 or tool 19.

To enable this action to be achieved, the hydraulic ram 22 is of theconstruction shown in FIG. 4. The piston assembly 23 of the ram 22includes a hollow piston rod 28 which carries at one end a piston 29provided with appropriate seals 30, the piston 29 being arranged forsliding movement within the cylinder 25. An annular shoulder 31 isformed inwardly of the "piston" end of the rod 28 against which anannular insert 32 abuts, the insert 32 being retained in position bymeans of a circlip 33. The insert 32 has its trailing end annularlychamfered as at 34 for a purpose as will be described further below.

The cylinder 25 extends from, and is secured to, the end block 26 bywelding or by other connection arrangement or alternatively the cylinder25 may be integrally formed with the end block 26. A hollow tubularmember 35 is rigidly attached to or integrally formed with the block 26and is arranged co-axially with the piston rod 28 and extends into thepiston rod 28 and piston 29. The hollow tubular member 35 carries at itsfree end, an annular valve member 36 having an annular chamfered endface 37 facing opposite the face 34 of the insert 32. The valve member36 is sealingly engaged with the insert 32 but is slidable relativethereto. The insert 32 is formed of a plastics material and when subjectto fluid pressure within the cylinder 25 it is forced towards theshoulder 31 so that it is compressed axially and caused to expandradially to be urged into firm engagement with the valve member 36 tomaintain sealing therebetween.

The co-operation between the valve member 36 and insert 32 provides afluid separation between the interior 38 of the piston rod 28 and theinterior 39 of the cylinder 25 until the piston rod 23 and piston 29 aremoved a predetermined distance into the cylinder 25 as governed by thedistance between the respective chamfered faces 34 and 37. When thispredetermined degree of movement occurs, the face 37 moves to a positionadjacent and past the face 34 permitting fluid communication between theinterior 38 of the piston rod 28 and interior 39 of the cylinder 25 asshown in FIG. 10. This also allows communication between the interior 39of the cylinder 25 and the interior of the hollow tubular member 35.

As stated above, the hollow tubular member 35 is connected to the block26 and as shown in FIGS. 5 and 6 communicates with a first uprightpassage 40 in the block 26 which at its upper end is connected through afurther passage 41 to a chamber 42, the latter opening to one side ofthe block 26 and being internally threaded at 43 so as to permitconnection via hydraulic hose to the hydraulic circuit of the primemover or a hydraulic accumulator. The chamber 42 communicates through apassage 44 and a second chamber 45 which has a first portion 46 with afurther portion 47 of enlarged diameter relative to the first portion46. A stepped piston assembly 48 (see FIG. 8) is located within thechamber 45 having connected pistons 49 and 50 located in the respectivechamber portions 46 and 47. The end 51 of the chamber portion 47 isblocked by a screw-in plug, whilst an air vent passage 52 vents thechamber portion 47 forwardly of the piston 50 to the atmosphere. Thearrangement of piston assembly 48 within the chamber 45 serves as anhydraulic pressure multiplier as described further below.

The chamber portion 46 additionally is connected via respective passages53, 54 and 55 to the interior 39 of the cylinder 25 with the passage 55opening into the cylinder 25 at 56. The piston assembly 48 also acts asa valve with fluid pressure in the chamber portion 47 normally forcingthe piston assembly 48 to one end of the chamber 45 (to the right asshown in FIG. 8) so as to block communication of the passage 44 with thechamber portion 46. In addition the piston 49 is forced hard up againstthe end of the chamber portion 46 to block communication of the passage53 with the chamber portion 46 and the passage 44. A further passage 57is connected to the passage 44 and passage 53, the passage 57 includinga valve seat 58 on which a ball valve 59 may seat. Further intersectingpassage 60 and 61 connect the chamber portion 47 rearwardly of thepiston 50 to the chamber 42.

In use, and where the chamber 42 is connected to a supply of hydraulicfluid, for example at 2000 psi, that fluid flows through the passages41, 61 and 60 to the chamber portion 47 to act on the piston assembly 48to urge the piston assembly 48 to the right as shown in FIG. 8 to causethe piston 49 to block off communication of the passage 44 with thechamber portion 46 and also be forced up against the end of the chamberportion 46. Thus, fluid flowing into the chamber 42 is blocked fromcommunication with the chamber portion 46 through the passageway 44because of the position of the piston 49. Supply of fluid to the chamberportion 47 causes, through the stepped nature of the piston assembly 45,a multiplying pressure effect at the end of the piston 49 proportionalto the respective surface areas of the pistons 50 and 49. For example,if, as in this instance, the pistons 50 and 49 have a ration of 3:1, thepressure seen at the end of the chamber 46 is three times the pressurein the chamber portion 47. Thus in the present case, under maximum load,the pressure at the end of the chamber portion is 6000 psi. when thepressure in the chamber portion 47 is 2000 psi. This pressure is alsoseen, because of the passages 53, 54 and 55 at the interior 39 of thecylinder 25. When an obstruction is encountered causing a force to beapplied to the tool shank 18, the piston 29, when the applied forceexceeds a predetermined level, will move within the cylinder 25 andtowards the end block 26 thus creating in the cylinder 25 a pressurewhich can reach, in this embodiment, up to three times the accumulatoror hydraulic system pressure.

In operation and where a very high resistance is encountered for examplewhere the shank 18 or tool 19 strikes a large obstruction, the pistonassembly 23 will be urged inwardly against the fluid pressure within thecylinder 25 as determined by the pressure multiplier piston assembly 48.As the piston 29 moves within the cylinder 25 towards the block 26overcoming that pressure, fluid will be caused to flow through thepassages 55, 54 and 53 to the end of the chamber portion 46 to act onthe piston 49. The piston assembly 48 will thus be forced to moverearwardly. Movement of the piston assembly 48 is permitted as thechamber portion 47 between the pistons 49 and 50 is vented through thepassage 52 to atmosphere. Communication between the chamber 46 andpassage 44 is opened when the piston assembly 48 moves a predeterminedamount permitting fluid in the interior 39 of the cylinder 25 to flowback to the accumulator or hydraulic system. Where a predeterminedmovement of the piston 29 occurs, the valve member 36 and insert 34 willbe moved to a position where the surfaces 34 and 37 are spaced apart topermit fluid to flow from the interior 39 of the cylinder 25 into themember 35 to pass into the passage 40. The passage 40, however, issubject to the reduced pressure through its connection via the passage41 to the chamber 42 which is connected to the hydraulic accumulator ornormal fluid pressure supply, in this case 2000 psi. This therebyreduces the pressure of fluid within the cylinder 25 which allows theshank 18 to have less resistance to movement and pivot upwardly andclear the obstructions in the manner shown in FIGS. 2 and 3.

Fluid at system pressure is continued to be supplied to the interior 39of the cylinder 25 through the tubular member 35 and past the insert 32and valve member 36 which are open, so as to act on the face of thepiston 29. Some fluid will also be supplied from the chamber 42 past thevalve 59 and through the passages 53, 54 and 55 to the interior of thecylinder 39. The pressure of the fluid supplied to the interior 39 ofthe cylinder 25 however is at the reduced system pressure, in thisinstance 2000 psi. The piston assembly 23 will thus be caused to extendfrom the cylinder 25 with the ram 22 expanding to urge the tool shank 18from the position of FIG. 3 back towards the positions of FIG. 2 andFIG. 1 with the tool 19 being forced into the ground. When however thepiston 29 has move a predetermined degree, the valve opening between thevalve member 36 and insert 32 will close thereby blocking fluid supplythrough the tube 35. Fluid however will continue to flow from thechamber 42 through the passage 44 and past the ball valve 59 into thepassages 53, 54 and 55 into the interior 39 of the cylinder 25 to act onthe piston 29 to continue extension of the piston assembly 23 andthereby urge the tool shank 18 back towards the position of FIG. 1.

As is apparent in FIG. 5 the ball valve chamber 57 has a seat 58 uponwhich the ball 59 seats under the biasing effect of a spring 62. Apassage 63 is connected between the valve seat 58 and chamber 44 tocommunicate the valve chamber 47 with the chamber 44. The passage 63 isof reduced diameter which limits the rate of flow of fluid from thechamber 44 to the passages 53, 54 and 55 to the interior 39 of thecylinder 25. This will reduce the speed of movement of the shank 18 backinto the ground thereby minimising shock loadings on the shank and othercomponents of the tool assembly 10. This overcomes a disadvantage ofconventional systems where to return a shank into the ground, the ram issubject to full flow from a hydraulic circuit causing rapid movement ofthe shank 18 into the ground and high shock loadings on the assembly.

The tool shank 18 is thus returned to the position of FIG. 1 and able toact in the manner described previously.

In the preferred embodiment movement of the tool is opposed by a fluidpressure ram using hydraulic fluids. Pneumatic rams however may also beused for opposing tool movement.

The soil penetrating tools as used in the tool assemblies describedabove may be of many different designs and include single or multiplesoil penetrating tips. In addition the tool may include multiple toolshanks whose movement from their operative attitude is opposed in themanner described.

All such variations and modifications are deemed to fall within thebroad scope and ambit of the invention as herein set forth.

I claim:
 1. A soil penetrating tool assembly, said tool assemblyincluding:a soil penetrating tool arranged for movement between alowered normal operating position and a raised position wherein saidtool can pass an obstruction, fluid ram means associated with said tool,said ram means comprising a cylinder and piston means movable in saidcylinder, fluid pressure relieving means associated with said ram means,said fluid pressure relieving means normally resisting displacement offluid from said ram means and thereby movement of said piston meansrelative to said cylinder and movement of said tool from said operatingposition, said fluid pressure relieving means permitting movement ofsaid piston means relative to said cylinder and thus said movement ofsaid tool when pressure of fluid within said ram means, consequent uponsaid tool encountering an obstruction, exceeds a predetermined maximumpressure, and valve means internally of said ram means, said valve meansconnecting said fluid ram means to a fluid supply having a pressurelower than said predetermined maximum pressure when said movement ofsaid piston means exceeds a predetermined movement whereupon said toolis permitted to move towards said raised position to pass saidobstruction.
 2. The soil penetrating tool assembly according to claim 1wherein said valve means includes a first valve part fixed againstmovement relative to said cylinder and a second valve part movable withsaid piston means.
 3. The soil penetrating tool assembly according toclaim 2 and including a fluid supply tube connected to said fluidsupply, said fluid supply tube extending longitudinally into saidcylinder from one end thereof, said valve means controllingcommunication between said tube and said cylinder.
 4. The soilpenetrating tool assembly according to claim 3 wherein said tube has afree end within said cylinder, said free end of said tube carrying saidfirst valve part of said valve means and wherein said second valve partof said valve means is adapted for cooperation with said first valvepart.
 5. The soil penetrating tool assembly according to claim 4 whereinsaid predetermined movement of said piston means relative to saidcylinder moves said valve parts out of register with each other topermit fluid flow therepast through said tube from said fluid supplyinto said cylinder.
 6. The soil penetrating tool assembly according toclaim 5 wherein said first valve part comprises an annular member orenlargement at said free end of said tube.
 7. The soil penetrating toolassembly according to claim 6 wherein said piston means is hollow toreceive said free end of said tube, and wherein said second valve partcomprises an annular insert within said hollow piston means.
 8. The soilpenetrating tool assembly according to claim 7 wherein said annularinsert is formed of plastics material, said insert when subject to fluidpressure within said cylinder expanding into sealing engagement withsaid first valve part when said valve means are closed.
 9. The soilpenetrating tool assembly according to claim 8 wherein said hollowpiston means comprises a hollow piston rod carrying a piston movablewithin said cylinder, said hollow piston rod receiving said free end ofsaid fluid supply tube.
 10. The soil penetrating tool assembly accordingto claim 3 and including an end housing adjacent said one end of saidcylinder, said end housing supporting said fluid supply tube andconnecting said fluid supply tube to said fluid supply.
 11. The soilpenetrating tool assembly according to claim 1 wherein said fluidpressure relieving means comprising a fluid pressure multiplierconnected to said fluid supply.
 12. The soil penetrating tool assemblyaccording to claim 11 wherein said fluid pressure multiplier comprises astepped piston having a larger piston end exposed to said fluid supplypressure and a smaller piston end communicating with said cylinder. 13.The soil penetrating tool assembly according to claim 12 wherein saidfluid supply is further connected to said cylinder through additionalpassage means, said passage means having a restricted area to restrictflow to said cylinder after said valve means is closed to return saidtool to said operating position at a reduced rate upon passing saidobstruction.
 14. The soil penetrating tool assembly according to claim 1wherein said tool is mounted for pivotal movement between its saidlowered and raised positions.
 15. The soil penetrating tool assemblyaccording to claim 14 and including mounting means for mounting saidtool assembly to a tool bar.
 16. The soil penetrating tool assemblyaccording to claim 15 wherein said tool is supported to said mountingmeans through linkage means.
 17. The soil penetrating tool assemblyaccording to claim 16 wherein said linkage means comprises a pivotallink assembly which is pivotally mounted at one end to said mountingmeans and which pivotally supports at its opposite end said tool. 18.The soil penetrating tool assembly according to claim 17 wherein saidfluid ram means is connected between said tool and said mounting means.19. The soil penetrating tool assembly, said tool assembly including asoil penetrating tool, support means for said tool, linkage meanspivotally connected to said tool and said support means for supportingsaid tool for movement between a lowered normal operating position and araised position wherein said tool can pass an obstruction, fluid rammeans pivotally connected between said tool and said support means, saidram means comprising a cylinder and a piston means movable in saidcylinder, fluid pressure relieving means associated with said ram means,said fluid pressure relieving means normally resisting displacement offluid from said ram means and thereby movement of said piston meansrelative to said cylinder and movement of said tool from said operatingposition, said fluid pressure relieving means permitting movement ofsaid piston means relative to said cylinder and thus said movement ofsaid tool when pressure of fluid within said ram means, consequent uponsaid tool encountering an obstruction, exceeds a predetermined maximumpressure, and valve means internally of said ram means, said valve meansconnecting said fluid ram means to a fluid supply having a pressurelower than said predetermined maximum pressure when said movement ofsaid piston means exceeds a predetermined movement whereupon said toolis permitted to move towards said raised position to pass saidobstruction.
 20. A fluid ram assembly, said ram assembly including acylinder and a piston means movable in said cylinder, fluid pressurerelieving means resisting displacement of fluid from said fluid ramassembly and thereby resisting movement of said piston means in saidcylinder from a first position unless pressure of fluid within said ramassembly exceeds a predetermined maximum pressure, and valve meansinternally of said ram assembly, said valve means connecting said ramassembly to a fluid supply having a pressure lower than saidpredetermined maximum pressure upon a predetermined movement of saidpiston means in said cylinder from said first position to allow a lessrestricted movement of said piston means in said cylinder away from saidfirst position.
 21. The fluid ram assembly according to claim 20 whereinsaid valve means includes a first valve part fixed against movementrelative to said cylinder and a second valve part fixed for movementwith said piston means.
 22. The fluid ram assembly according to claim 21and including a fluid supply tube connected to said fluid supply, saidfluid supply tube extending longitudinally into said cylinder, saidvalve means controlling communication between said tube and saidcylinder.
 23. The fluid ram assembly according to claim 22 wherein saidtube has a free end within said cylinder, said end carrying said firstvalve part of said valve means and wherein a second valve part of saidvalve means is arranged on said piston means for movement therewith andadapted for cooperation with said first valve part.
 24. The fluid ramassembly according to claim 23 wherein movement of said valve parts outof register with each other permits fluid flow therepast from said fluidsupply into said cylinder upon said predetermined movement of saidpiston means.
 25. The fluid ram assembly according to claim 23 whereinsaid first valve part comprises an annular member of enlargement at saidfree end of said tube.
 26. The fluid ram assembly according to claim 25wherein said piston means is hollow to receive said free end of saidtube, and wherein said second valve part comprises an annular insertwithin said piston means.
 27. The fluid ram assembly according to claim26 wherein said annular insert is formed of plastics material, saidinsert when subject to fluid pressure within said cylinder expandinginto sealing engagement with said first valve part when said valve meansare closed.
 28. The fluid ram assembly according to claim 27 whereinsaid hollow piston means comprises a hollow piston rod carrying a pistonmovable within said cylinder, said hollow piston rod receiving said freeend of said tube.
 29. The fluid ram assembly according to claim 21wherein said fluid pressure relieving means comprises a fluid pressuremultiplier connected to said fluid supply.
 30. The fluid ram assemblyaccording to claim 29 wherein said fluid pressure multiplier comprises astepped piston having a larger piston and exposed to said fluid supplypressure and a small piston end communicating with said cylinder. 31.The fluid ram assembly according to claim 30 wherein said fluid supplyis connected to said cylinder through additional passage means, saidadditional passage means having a restricted area to restrict flow tosaid cylinder after said valve means is closed to return said pistonmeans to said first position at a reduced rate.
 32. A soil penetratingtool assembly, said tool assembly including:a soil penetrating toolarranged for movement between a lowered normal operating position and araised position wherein said tool can pass an obstruction, fluid rammeans associated with said tool, said ram means comprising a cylinderand piston means movable in said cylinder, fluid pressure relievingmeans associated with said ram means, said fluid pressure relievingmeans normally resisting displacement of fluid from said ram means andthereby movement of said piston means relative to said cylinder andmovement of said tool from said operating position, said fluid pressurerelieving means permitting movement of said piston means relative tosaid cylinder and thus said movement of said tool when pressure of fluidwithin said ram means, consequent upon said tool encountering anobstruction, exceeds a predetermined maximum pressure, a fluid supplytube connected to a fluid supply having a pressure lower than saidpredetermined maximum pressure, said fluid supply tube extendinglongitudinally into said cylinder from one end thereof, and valve meansinternally of said ram means, said valve means connecting said ram meansto said fluid supply through said fluid supply tube when said movementof said piston means exceeds a predetermined movement whereupon saidtool is permitted to move towards said raised position to pass saidobstruction.
 33. A fluid ram assembly, said ram assembly including acylinder and a piston means movable in said cylinder, fluid pressurerelieving means resisting displacement of fluid from said fluid ramassembly and thereby resisting movement of said piston means in saidcylinder from a first position unless pressure of fluid within said ramassembly exceeds a predetermined maximum pressure,valve means internallyof said ram assembly, a fluid supply tube connected to said fluidsupply, said fluid supply tube extending longitudinally into saidcylinder, and said valve means controlling communication between saidtube and said cylinder and connecting said cylinder through said tube tosaid fluid supply upon a predetermined movement of said piston means insaid cylinder from said first position to allow a less restrictedmovement of said piston means in said cylinder away from said firstposition.
 34. A soil penetrating tool assembly, said tool assemblyincluding a soil penetrating tool, support means for said tool, linkagemeans pivotally connected to said tool and said support means forsupporting said tool for movement between a lowered normal operatingposition and a raised position wherein said tool can pass anobstruction, fluid ram means pivotally connected between said tool andsaid support means, said ram means comprising a cylinder and a pistonmeans movable in said cylinder, fluid pressure relieving meansassociated with said ram means, said fluid pressure relieving meansnormally resisting displacement of fluid from said ram means and therebymovement of said piston means relative to said cylinder and movement ofsaid tool from said operating position, said fluid pressure relievingmeans permitting movement of said piston means relative to said cylinderand thus said movement of said tool when pressure of fluid within saidram means, consequent upon said tool encountering an obstruction,exceeds a predetermined maximum pressure,a fluid supply tube connectedto a fluid supply having a pressure lower than said predeterminedmaximum pressure, said fluid supply tube extending longitudinally intosaid cylinder, and valve means internally of said ram means, said valvemeans connecting said fluid ram means to said fluid supply through saidfluid supply tube when said movement of said piston means exceeds apredetermined movement whereupon said tool is permitted to move towardssaid raised position to pass said obstruction.